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Sylvia Has Two Partners

Like tiny planets in a miniature solar system, two chunks of rock have been found orbiting an asteroid known as Sylvia. The discovery is the first observation of a triple-asteroid system and may give clues to the nature of the process that spawned the planets some 4.6 billion years ago.

Asteroids--rocky bodies circling the sun between the orbits of Mars and Jupiter--are thought to be leftovers from the formation of the solar system. Measuring 380 by 260 by 230 kilometers, Sylvia (discovered in 1866 and named after Rhea Sylvia, the mythical mother of the founders of Rome) is the ninth largest. Over the past decade, some 20 asteroids have been found to be accompanied by one satellite. Planetary scientists assume that these small moons are the results of collisions in the asteroid belt: Rocky debris from a collision reaccumulates into a so-called rubble pile asteroid, sometimes with a smaller object in a stable orbit around it.

Sylvia is now the first asteroid known to have two satellites. In 2001, astronomers spotted the first satellite--a rock 18 kilometers in diameter that orbits the asteroid every 3.65 days. But it took the powerful 8.2-meter Very Large Telescope of the European Southern Observatory in Chile and a special camera designed to compensate for atmospheric turbulence to spot Sylvia's second partner. The new moonlet, discovered by planetary scientist Franck Marchis of the University of California, Berkeley, and colleagues, is just 7 kilometers across and orbits Sylvia every 33 hours.

From the observed orbits of the two satellites, the team deduced that Sylvia's density is just 20% higher than that of water, suggesting that the asteroid is very porous. This is in excellent agreement with the collision theory, which predicts that many asteroids are loose aggregates of smaller fragments, the researchers report 11 August in Nature. Future measurements may shed light on the physical properties of the satellites themselves.

"To find a system with two small satellites is quite exciting," says asteroid researcher Daniel Durda of the Southwest Research Institute in Boulder, Colorado. Indeed, Durda's computer simulations had suggested that it is much more likely for asteroids to end up with only one moonlet, but "this relatively early discovery of a true trinary system hints that we will find more," he says. Finding more multisatellite asteroids should force astronomers such as Durda to improve their models, which ultimately would lead to a better understanding of the solar system's dynamic past.